Activists said the Syrian regime was responsible for the killings of at least 70 people -- 10 children among the dead -- leading the United Nations to replace a scheduled Security Council session for Wednesday morning with an emergency meeting.
Syrian President Bashar al-Assad's military denied using chemical weapons and blamed rebels for the carnage. Russia, Syria's strong ally, said it had no warplanes in the vicinity.
But from Washington to London to Jerusalem, leaders denounced the reported airstrike, which if proven true would be one of the deadliest chemical attacks in Syria in years.
"Today's chemical attack in Syria against innocent people, including women and children, is reprehensible and cannot be ignored by the civilized world," US President Donald Trump said, adding that he thinks the attacks was a consequence of the past administration's weakness and irresolution. "President Obama said in 2012 that he would establish a 'red line' against the use of chemical weapons and then did nothing."
UK Foreign Secretary Boris Johnson, speaking at a news conference with his German counterpart, had harsh criticism of the Syrian government.
"If this were to be proved to have been committed by the Assad regime then it would be another reason to think they are an absolutely heinous outfit, that is, it is a war crime," he said.
Doctor cannot shake horrible sights
Airstrikes hit the rebel-held city of Khan Sheikhoun in Idlib province on Tuesday morning, giving off a "poisonous gas," according to Anas al-Diab, an activist with the Aleppo Media Center.
Many casualties were the result of asphyxiation, doctors said. Videos on social media purporting to be from the scene show people, including children, who appear unresponsive; others struggle to breathe or wear oxygen masks.
A doctor in a hospital close to Khan Sheikhoun told CNN: "Today around 7:30 a.m., about 125 ... arrived to our hospital. Twenty-five of them were already dead, 70% to 80% of the wounded people were kids and women.
"The symptoms were pale skin, sweating, narrow or pin-eye pupils, very intense respiratory detachments. Those symptoms match the usage of sarin."
The doctor, who cannot be named for security reasons, said tests were needed to identify whether sarin gas, a man-made nerve agent, was to blame.
Dr. Fares al-Jundi rushed to the hospital. He estimated there were 500 wounded people. They covered the floors of the entire hospital, from the patients' rooms to the operating rooms and the corridors.
The doctor said whole families were killed. They died of asphyxiation; foam covered their mouths. Many died suddenly, he said. "I believe this horrible memory will stay with me for the rest of my life."
Chaos, confusion at the scene
Hadi al Abdullah, a citizen journalist, arrived at the scene two hours after he says a single gas rocket hit the town.
Abdullah, who had common side effects of a nerve gas attack -- blurred vision, a splitting headache and lethargy -- told CNN the scene was chaos. Families died in their homes, and in the streets, children risked exposure looking for their parents.
At first, people were unaware it was a gas attack, Abdullah said. People who rushed to help others were overtaken by the gas, including several White Helmets, who had to be rescued by others from the group, also known as the Syrian Civil Defense.
The Aleppo Media Center put the death toll at 70 but there were different numbers from other groups. The UK-based Syrian Observatory for Human Rights said at least 58 were dead, including 10 children. The High Negotiations Committee, an umbrella opposition group, said as many as 100 died.
Syrian military denies responsibility
The Syrian army "categorically denies the use of chemical and poisonous material in Khan Sheikhoun," the General Command said in a statement.
"The Syrian army holds the terrorist groups and those supporting them responsible for the use of chemical and poisonous material and for the careless wasting of innocent civilians' lives to achieve their despicable goals and agendas," the statement said.
In an apparent reference to the crisis, two state-run Syrian media outlets earlier reported an explosion at a "rebel poison gas factory" in the countryside of Idlib province, implying that it was an accident related to rebels building chemical weapons. Idlib province is largely controlled by an alliance of rebel forces and is regularly targeted in airstrikes by the Syrian military and its ally, Russia.
The Russian Defense Ministry said none of its jets carried out airstrikes in the area, according to a ministry statement carried by state-run news agency Tass.
The Syrian Coalition, an umbrella opposition group, referred to the suspected chemical attack as a "crime similar to that in Eastern Ghouta in 2013 that the international community allowed to pass without accountability or punishment."
The tweet was in reference to a 2013 chemical attack on the Damascus suburb of Ghouta, which was blamed on the Syrian regime and prompted US President Barack Obama to threaten military action.
If Tuesday's reports of a chemical attack by the regime are confirmed, experts say it could be the biggest since Ghouta.
US Secretary of State Rex Tillerson called on Russia and Iran "to exercise their influence over the Syrian regime" and that the two nations "bear great moral responsibility for these deaths."
McCain: Assad must pay a price
US Sen. John McCain called for President Donald Trump to send a strong signal to Assad that the attack will not be tolerated.
Referring to the 2013 chemical attack, McCain told CNN: "We've seen this movie before, it was when Barack Obama said they would have a red line, they crossed it and he did nothing. Bashar Assad and his friends, the Russians, take note of what Americans say."
The Ghouta attack prompted Obama to ask Congress to authorize military action against Syria in 2013. Shortly after, Syria agreed to a Russian proposal to give up control of its chemical weapons, leading Obama to retreat from threats of military action.
Analyst: Regime may increase attacks
Haid Haid, a Syrian columnist and researcher, told CNN that he thinks the United States is "not interested in any way challenging the rule of Assad."
"The regime has been systematically using small scale (chemical) attacks over a long time in order to normalize the use of those attacks," said Haid, an associate fellow at Chatham House, an international policy think tank based in London.
"Once the regime is sure that the attacks won't trigger any sort of reaction from the international community, I believe the regime will start increasing those attacks."
Since the civil war began in 2011, an estimated 400,000 Syrians have been killed, according to figures issued by the United Nations last year.